text:
What are the best places to gain the most altitude in a glider?
I'm a beginner pilot and I started with gliding. How to spot the best thermals and what is the most efficient way to climb?

Follow the birds. Circle where they are circling.  Upwind side of a mountain, or a ridge. Large dark areas on the ground.  Also check with other glider pilots in your area for good local spots.

Welcome to aviation.se! Welcome to Stack Overflow! Please take the tour, have a look around, and read through the help center, in particular How do I ask a good question? and What topics can I ask about here?. -- Answeres to your question should be part of your glider pilot training.

First, you need practice. Take every opportunity to expand your experience. Theory helps but will never compensate for missing practical experience.
But then you also need to know how thermals form. Let's start low, close to the ground. Two things are important: A chance for the sun to heat the ground (and, consequently, the air above), and an opportunity for that heated air to ascend. Dry ground will heat up better than moist ground, and dark earth better than light sand. If the ground is completely featureless, the air will stay on the ground. So you need to find the trigger which determines where air starts to rise up. Once it does this, more air will be sucked in from the side, and will follow up. That is how a thermal starts.
And that summer gust, which is the horizontal motion of more warm air flowing towards the point where a thermal starts, should indicate to you that now you have a chance to stay up, so now you need to get into your glider. Early in the day the thermals will not go high, so don't be impatient. On the positive side, there are many thermals and a good chance of finding the next one quickly.
Once in the air, look for triggers. One of the easiest is a windward edge of a forest. Warm air is driven towards the forest by the prevailing wind, and the trees will push the air up, kickstarting its ascent. Fly along and a bit leeward of the ridge and you can't miss the thermals there. But even a closed forest is a good source of thermals: The dark trees heat up well and the air between the trees heats up quickly. A small elevation is normally enough to start a thermal. In the mountains, a windward ridge is also an obvious trigger, and it is advisable not to drop below the height of the lowest ridge, as you suddenly might find yourself unable to find any more thermals.
Once higher up the ground features become more and more unimportant. For one, wind is shifting everything sideways, but also now the large-scale vorticity which forms the clouds dominates where the thermals form. Think of cumulus clouds as giant thermal vacuum cleaners which collect the rising air from all around. But not any cumulus cloud: Most promising are the ones which grow, have sharply defined edges and a dark bottom which is bulged upwards towards the center. Flee from the fuzzy ones with a round bottom: They have lost their pulling power and are about to disappear.
Why the bulged-in bottom? THAT is where the heated air goes up and pushes the bottom of the cloud up. But what goes up must come down: In the vicinity of the best clouds you will also find the worst downdrafts. Fly through those quickly and pull up as soon as the downdraft stops.
If you fly cross-country, the most important knowledge is in which direction the wind is blowing. If the wind is strong enough (more than a soft breeze), thermals will line up in wind direction, so you can just leave one straight into the wind (or with the wind) and be sure to hit the next one shortly. This works particularly well if there are no clouds to follow (blue thermals). Just remember where exactly that one-before-last thermal was and fly off in exactly the opposite direction. When that brings you too far off the planned course, cross the valley of downdrafts orthogonal to the wind direction until you arrive in the next lineup of thermals.
Now to how to circle properly. A glider will fly at its minimum sink speed when induced drag is three quarters of total drag - given the high aspect ratio of gliders this means at a really high lift coefficient. Close to stall. Therefore, it is most important to learn how the glider warns you that you are about to stall. Also, some gliders will happily let you fly in a stalled condition (the Discus is a prime example) with really poor sink speed but without loss of control. Go up to maybe 1000 m and then stall the glider slowly. Learn how that feels. Then continue with circles. Stall the glider again. At some point you will feel how the stick forces lighten (that is when flow separation starts on the ailerons!) and you will learn to fly just a bit faster than that. Similarly, the stalled Discus has really soft and mushy controls - fly faster until the controls feel stiff again!
Since circling with more than 45° of bank is rather stressful, fly between 30° and 45° of bank angle. You need to catch the core of the thermal, where the climb speed is highest, and banking more is worth the tradeoff in many cases. Tighten the circle when the climb speed decreases and widen it when speed increases - that helps you to center your circling with the center of the thermal. Learn the hysteresis of your instruments - maybe you need to phase-shift your reaction so you really land in the center of the thermal. For me, I found it easier to remember where the climb speed was best and to shift my next circle in that direction when flying two-seaters - they are too sluggish to do that on-the-fly optimization you can do with the more nimble single seaters.

If there are cumulus clouds, you hunt for the thermals feeding them on the upwind side.  Once you locate one, you "map" its location in your mind by circling and noting which quadrants have the strongest climb and which have sink, shifting your circle toward where you think the core of the thermal is by straightening out momentarily.  You are in the sweet spot when you have a decent climb rate all the way around.   You fly just above the stall in the thermal, roughly at minimum sink speed.
If the air is too dry so there are no clouds to mark thermal tops, you have to troll for them.
If there are soaring birds or other gliders circling, well, it's obvious.


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What is the type of this plane?
I tried looking with aircraft tag on engine but no answers. I just need to find the type of this jet.

Its a Bombardier Challenger 600 (CL-600-2A12) as per the op-specs of this 135 charter

You can easily find this out by searching for the tail number, N97SG, which is analogous to the license plate of a car.  It's a Bombardier Challenger 600 series business jet.

source: Stack Exchange

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sort: votes


text:
Why can military aircraft descend below the DA / MDA during an instrument approach?
According to §91.175(c)

Except as provided §91.176 of this chapter, where a DA/DH or MDA is applicable, no pilot may operate an aircraft, except a military aircraft of the United States, below the authorized MDA or continue an approach below the authorized DA/DH unless—

So why are military aircraft exempt from this restriction?

The most simple reason is the military has its own set of flight rules (AFI 11-202, OPNAVINST 3710.7U) that for many years duplicated many of the applicable FAR/AIM regulations. In addition, the varied mission sets and training scenarios the military operates under sometimes require them to do non-standard things, so there are a number of "carve-outs" for them to perform and regulate their own maneuvers. In general, any exception to these rules are rare, unless specifically laid out in an instruction, and likely to be used only in times of national emergency or operational necessity.
One example of a reason to descend below an MDA or DA would be when shooting a self-contained approach. A self-contained approach (SCA) or IPRA (Independent Precision Radar Approach) is one where the aircraft uses its own navigational capability (INS/Radar/Radio Altimeter) to guide the aircraft to the runway without ground-based navigational aids. Referencing the old AFI 11-202v3 AFSOC Sup, it details when constructing SCAs,

7.4.6.5 For AFSOC SCAs and IPRAs, the MDA/DA will be commensurate with aircraft equipage capabilities and mission requirements. The applicable obstacle clearance will be maintained until descent on the final approach segment.

While practice requirements for SCAs require good weather, operational weather minimums will be much lower, and specified by the MDS-specific operations procedure. This may in some cases allow an aircraft to shoot an approach to a runway and descend below any MDA/DA on an existing approach, based on on-board capabilities.

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Where can I purchase air diffusers
Where can I purchase the air diffuser located in the overhead compartment of each passenger aircraft seat? I need them for a non aviation application. Thanks

Seems easy enough to mock-up with a 3D printer.

The technical name appears to be "Eyeball Air Vent". A Google search turned up a number of hits.
Here's an aluminum product from one manufacturer:

Here's a plastic model more in line with what you'd see on an airliner, from a different seller:

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text:
Are commercial aircraft designed with a shared data network?
In modern airliners, do systems like the in-flight entertainment system (IFE) share the same physical and/or logical network as the flight management systems (FMS)? Is there any communication between the IFE and the FMS? Also, is there any Airbus training or procedure that might deal with what to do if flight systems were being tampered with?
I'm asking because an information security researcher named Chris Roberts is in a bit of trouble with the FBI after claiming he hacked into the entertainment systems on Airbus and Boeing airliners and used the access to tamper with the airplanes' flight control systems. Apparently he would connect into a Seat Entertainment Box (SEB) and hack the IFE system. I am an information security professional myself and I have no reason to disbelieve this part given my personal experience.
What I am initially skeptical about is that he has claimed to have successfully tampered with flight control systems using the access gained from the compromised IFE. Specifically listed in this document he has allegedly claimed that he issued a climb command to one of the engines through the thrust management computer, causing it to spool up and temporarily cause the aircraft to yaw. This would only be possible if the IFE were able to communicate somehow with the FMS.

I've read that some people doubt the story entirely since the IFE is apparently connected to a read-only bus of the actual flight computers. Like many times in aviation it's a good idea to let the facts be presented and verified before jumping at conclusions :)

This doesnt answer your question but you should look into the vulnerabilities of nextgen air traffic control there was a defcon talk about this and I would bet that is how he manipulated the aircraft. I find it hard to believe that the IFE would be so easily linked to critical flight systems (but I've been wrong before)...

@MatthewPeters I've read that article and watched the defcon talk.  I see no relationship between those and this question.

If FMS is pwned, the best response is most likely a prayer.

@Simon, GdD is assuming the news article is reporting the facts correctly in that the researcher issued a command via the IFE. I am merely suggesting that (in accordance with the actual research) the researcher in question merely manipulated the vulnerabilities with the NextGen system. Either way it is irrelevant speculation because GdD is asking the (currently provable) question of if the IFE and FMS are networked and not the (currently unprovable) question of how...

Note I am skeptical on the ability to issue commands from a compromised IFE, however I've learned never to discount the possibility of bad design, hence the question. If I had to bet on it I would put my money against the possibility.

Aircraft do have LAN Networks for their sensors to minimize wiring (and consequently weight), however I would assume these are closed circuit networks and NOT connected to the IFE system.
In terms of the FMS system on aircraft according to this they do have databases however I dug into it a bit and it seems that based on this walk through on updating you need to physically connect to the device to change that database (for Rockwell Collins Equipment). It also seems that the Honeywell FMS computers have a similar way of updating via USB. While I cant say for certain it does not seem the FMS is on any kind of network aside from its own.
The big question is autopilot/FBW systems and can they be overridden or tampered with. It seems that Boeing-Honeywell has a patent on some remote flying systems but has not yet implemented it on any aircraft. I am still looking for some solid evidence that the systems have a point of access on the network. It is clear that they can be altered if you can physically connect to them for update purposes. Keep in mind you would have to load them with false data.
This seems to be the autopilot for the 777 however Rockwell Collins does not seem to allow you to get the info on it very readily. Maybe someone else on here can comment on how its set up.
This nice paper overview of FBW systems on a 777, in section 11.4.4 lists the connections the FBW system has to other systems on the plane. While this is by no means an official document (at least I don't think so) it does not list the IFE system as a connection and is pretty clear on the fact that everything connects over the ARINC 629 style data busses.
Edit:
It does appear that ethernet technology is making its way onto aircraft however it seems that the tech still uses ARINC Specification 664. However page 2 of this spec documents seems to imply that through a "gateway" the system is connected to the "internet". However Page 38 clearly states

Although AFDX/ARINC 664 is a closed network, UDP port numbers
  should be selected from the Dynamic/Private range of numbers. The reason
  for this is that there could be potential conflicts with the standard port
  number assignments when a gateway is used to communicate between the
  AFDX network and the Internet

So it is connected to the internet but the flight control systems are on a closed network. They also advise you to use the private range of ports to avoid any possible conflicts. Here is another good paper on the technology. It does seem that they are moving towards some internet like devices on aircraft but it also seems that they will remain closed networks. Companies are looking to consume some modern tech for an age old application as its simpler and easier to use existing high speed bus technology than reinvent the wheel and make a new custom bus. Just because something has a router and ethernet cables does not mean its connected to the internet!

I don't know for sure that they are currently in use, but Ethernet networks including both the flight control and IFE systems are at least planned for the near future.

@fooot -- that link is in reference to AFDX/ARINC 664, basically

Note, that IFE definitely gets data from the avionics (so it can draw the map with current position and show current speed). It probably also does send some data back to the avionics, namely it reports faults so they can show up on the ECAM display in cockpit and be recorded for maintenance. And then bug in the gateway might allow faking some important data.

@Jan I did notice on one of the diagrams in the links there that there was a labeled data output for the maps on the IFE displays but no other detail on that connection was provided in the document so I did not want to speculate if it was a one way or full duplex bus. I was unaware the IFE issue would show up on the screens in the cockpit though.

@Dave: I don't know. But I would expect that at least in some variants it can. Of course the content wouldn't show up on cockpit screens. But there is usually some system that collects status reports from all systems for maintenance to know what to fix and I would expect the IFE to send it's status there too.

@Jan, Interesting, Im still looking for some comprehensive documentation on the setup but its proving hard to find or hard to get a hold of. Next time I am down at the airport flying I plan to swing by the maintenance bay and see if anyone I know there can shed light on this mystery.

The Register reported on this subject
The links between the In-Flight-Entertainment (IFE) network and the Engine Indicating and Crew Alerting System (EICAS) is one-way. EICAS provides IFE with location and speed for display on maps.

"IFE systems on commercial airplanes are isolated from flight and navigation systems," Doug Alder, spokesman for Boeing Commercial, told El Reg. "While these systems receive position data and have communication links, the design isolates them from the other systems on airplanes performing critical and essential functions."

...

At last year’s DEFCON hackers' meeting Dr Phil Polstra, professor of digital forensics at Bloomberg University (and a qualified commercial pilot and flight instructor), delivered a lecture on the feasibility of inflight aircraft hacking. It turns out it’s a lot more difficult than you might think.

...

IFE systems do receive some information from the engine-indicating and crew-alerting system (EICAS), chiefly the aircraft’s location and speed for those little progress maps, but this data comes through a unidirectional Network Extension Device (NED).
The EICAS gets its data from a hydromechanical controller which has an electronic interface, known as an EEC, and Roberts appears to be claiming that he overrode the EEC on one engine to produce the change of course.
"In order for this to actually happen he would need to be on a plane where the levels are not directly connected to the EEC, successfully compromise the EICAS, get the EEC to accept input from the EICAS (recall this is really a monitoring system), send a bogus mode change to climb, and somehow prevent the throttle quadrant from immediately sending a correct command," he told the Reg.
"On top of this long list of conditions there are two basic facts that would make his actually flipping a plane on its side unlikely. First, jet engines do have a spool up time so this wouldn’t happen immediately. Second, the second a pilot touches the levers new signals will be sent to the EEC. When you put this all together it seems unlikely that Chris was able to ‘take over’ this plane as he claimed."

"In order for this to actually happen he would need to [...]", and then a list of actions that are under the control of software. All that is working as long as the controlling firmware / programs are not overridden. If hackers can change these software, then they can do plenty of things without control, as they do with smart cars. Cryptographic keys exchange can protect further each connected device against such action, but this has also been hacked in several cases.

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Why is TSO named so? Where is the 'order' part in this? Isn't it just a standard?
Wiki says: A Technical Standard Order (TSO) is a minimum performance standard issued by the United States Federal Aviation Administration for specified materials, parts, processes, and appliances used on civil aircraft.
I am unable to figure out why is it named Technical Standard Order. Does order imply it is a mandatory standard to be met?


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When was the first picture of the Wright brothers' 1903 plane published?
In their January 6, 1904, press release, the Wright brothers said they flew 4 times on December, 17, 1903, and gave a short description of each trial (see the article below). However, the text ends with this statement: "we do not feel ready at present to give out any pictures or detailed description of the machine".
The question is: When did the Wright brothers made available to the public at least a photo showing their 1903 airplane?
“Wright Flyer. A Report of Late Tests Is Given by Messrs. Wright, Inventors of the Machine.”, Dayton Press, Ohio, US, January 6, 1904.

Wright Flyer
A Report Of Late Tests
Is Given by Messrs. Wright, Inventors of the Machine.
Interesting Description of the Trials Made at Kitty Hawk.
It had not been our intention to make any detailed public statement
  concerning the private trails of our power “Flyer” on the 17th of
  December last; but since the contents of a private telegram,
  announcing to our folks at home the success of our trials, was
  dishonestly communicated to newspaper men at the Norfolk office, and
  led to the imposition upon the public by persons who never saw the
  “Flyer” or its flights, of a fictitious story incorrect in almost
  every detail; and since this story, together with several pretended
  interviews or statements, which were fakes pure and simple, have been
  very widely disseminated, we feel impelled to make some corrections.
  The real facts were as follows:
On the morning of December 17, between
  the hours of 10:30 o’clock and noon, four flights were made, two by
  Orville Wright and two by Wilbur Wright. The starts were all made from
  a point on the level sand about 200 feet west of our camp, which is
  located a quarter of a mile north of the Kill Devil sand hill, in Dare
  county, North Carolina. The wind at the time of the flights had a
  velocity of 27 miles an hour at 10 o’clock, and 24 miles an hour at
  noon, as recorded by the anemometer at the Kitty Hawk weather bureau
  station. This anemometer is 30 feet from the ground. Our own
  measurements, made with a hand anemometer at a height of four feet
  from the ground, showed a velocity of about 22 miles when the first
  flight was made, and 20½ miles at the time of the last one. The
  flights were directly against the wind. Each time the machine started
  from the level ground by its own power alone with no assistance from
  gravity, or any other sources whatever. After a run of about 40 feet
  along a mono-rail track, which held the machine eight inches from the
  ground, it rose from the track and under the direction of the operator
  climbed upward on an inclined course till a height of eight or ten
  feet from the ground was reached, after which the course was kept as
  near horizontal as the wind gusts and the limited skill of the
  operator would permit. Into the teeth of a December gale the “Flyer”
  made its way forward with a speed of ten miles an hour over the ground
  and 30 to 35 miles an hour through the air. It had previously been
  decided that for reasons of personal safety these first trials should
  be made as close to the ground as possible. The height chosen was
  scarcely sufficient for maneuvering in so gusty a wind and with no
  previous acquaintance with the conduct of the machine and its
  controlling mechanisms. Consequently the first flight was short. The
  succeeding flights rapidly increased in length and at the fourth trial
  a flight of 59 seconds was made, in which time the machine flew a
  little more than a half mile through the air, and a distance of 852
  feet over the ground. The landing was due to a slight error of
  judgment on the part of the operator. After passing over a little
  hummock of sand, in attempting to bring the machine down to the
  desired height, the operator turned the rudder too far, and the
  machine turned downward more quickly than had been expected. The
  reverse movement of the rudder was a fraction of a second too late to
  prevent the machine from touching the ground and thus ending the
  flight. The whole occurrence occupied little, if any more, than one
  second of time. Only those who are acquainted with practical
  aeronautics can appreciate the difficulties of attempting the first
  trials of a flying machine in a 25 mile gale.
As winter was already
  well set in, we should have postponed our trails to a more favorable
  season, but for the fact that we were determined, before returning
  home, to know whether the machine possessed sufficient power to fly,
  sufficient strength to withstand the shock of landings, and sufficient
  capacity of control to make flight safe in boisterous winds, as well
  as in calm air. When these points had been definitely established, we
  at once packed our goods and returned home, knowing that the age of
  the flying machine had come at last.
From the beginning we have
  employed entirely new principles of control; and as all the
  experiments have been conducted at our own expense, without assistance
  from any individual or institution, we do not feel ready at present to
  give out any pictures or detailed description of the machine."

According to The Wright brothers’ patents (full text) and their importance for aviation (see the three citations below):

the first picture of the 1903 plane was published in September 1908 in The Century Magazine together with the first pictures of the 1904 and 1905 machines,
the first clear images showing a Wright airplane appeared in print on August 12, 1908, in French newspapers,
the first ever photos, unfortunately unclear, of a Wright airplane were published simultaneously by Collier's and The Scientific American on May 30, 1908.

(1)

" Pictures claimed by Orville Wright as made between December 17,
  1903, and October 5, 1905, and showing three different planes (the
  1903, 1904 and 1905 models) first appeared in print quite late, in
  “The Wright Brothers’ Aeroplane” by Orville and Wilbur Wright (The
  Century Magazine, New York, September 1908, Vol. LXXVI, No. 5, pp.
  641-650). "

(2)

“No technical drawing, detailed description or clear picture showing a
  Wright plane, on the ground or in the air, were made available to the
  general public before August 8, 1908, so none of the powered
  apparatuses constructed and flown before the above mentioned date,
  according to what the two inventors pretended, could have been a
  source of inspiration for other aviation pioneers because nobody knew
  exactly what those machines looked like. The French newspapers (see
  the examples listed below) started to show pictures of Wilbur’s
  biplane on August 12, 1908.
1908-08-12, “De nouveau, Wilbur Wright à volé”, L’Auto, Paris, August
  12, 1908, col. 3-4, p. 1.
1908-08-12, Raoul Sabatier, “L’homme volant. Wilbur Wright a fait
  hier à 25 mètres de hauteur plus de Quatre kilomètres en 3 minutes 43
  secondes.”, Le Journal, Paris, August 12, 1908, col. 3-4, p. 1.
1908-08-12, “Les expériences de Wright. Supériorité de l’aéroplane
  américain.”, Le Petit Parisien, Paris, August 12, 1908, col. 3-4, p.
  2.
1908-08-12, “La conquête de l’air. L’Aéroplane Wright.”, Le Radical,
  Paris, August 12, 1908, col. 3-4, p. 2.
All these papers contain at least one clear photo showing the Wright
  machine. “

(3)

“ Another note would be about the May 1908 alleged flights and
  authentic pictures showing a Wright powered machine in the air. These
  photos, published on May 30, 1908, and taken by James H. Hare from the
  American magazine Collier’s, do not reveal anything new. They are
  apparently shot from a great distance and show a slightly modified
  1902 glider. The engine and propellers are not visible. In their May
  30, 1908, issues, Collier’s published one such image and the
  Scientific American two, in the following articles:
1908-05-30, Arthur Ruhl, “History at Kill Devil Hill. A Description
  of the First Flight of the Wright Brothers’ Aeroplane Witnessed by an
  Uninvited and Impartial Jury Representing the World at Large.”,
  Collier’s, New York, May 30, 1908, Vol. XLI, No. 10, pp. 18-19 and
  26.
1908-05-30, “The Wright Aeroplane Test in North Carolina”, Scientific
  American, New York, May 30, 1908, Vol. XCVIII, No. 22, cover and p.
  393. "

Source: The Wright brothers' patents - a book about all patents and some articles of the two American inventors.

@quietflyer, Read the paragraph starting with "A close study of the photographs which we reproduce shows ..." (the middle column of the May 30, 1908, article in the Scientific American). The text says the flying machine in the picture has a motor and propellers.

Sorry, I was clearly wrong to believe the photo showed the aircraft sitting on the hill. Fantastic research there.

A quick google search of "first published photo Wright flyer" reveals that the iconic photo of the 1903 Flyer in flight was first published in 1908.  This photo was in possession of the Wrights and this publication date undoubtedly corresponds to their actual release of the photo for publication.  All photos of the 1903 Flyer were in possession of the Wrights and it seems unlikely that they would have released any of these photos for publication before this date, since that would only reveal technical details of construction-- which they were trying to keep under wraps-- without even establishing proof of flight.  (This photo, of the fourth flight, would be one possible exception to that observation, since the quality is so poor.)
Many photos were also taken by the Wrights or by persons authorized by them of the 1904 Flyer II and the 1905 Flyer III at Huffman Prairie near Dayton Ohio.
Here is one website that shows many photos of the Wright aircraft from the 1903 to 1905 time period, as well as later.

That's a great website!  Got that bookmarked :)


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How is the fuselage of the Boeing 787 produced?
I am particularly interested in the Boeing 787 Fuselage because it is one of the first aircraft to carry such a high percentage of Carbon Fibre Reinforced Polymer (CFRP).
Can someone explain how the fuselage is made? I know the basics of making
CFRP which include fibre being wrapped onto in mandrel in different orientations then heating in an autoclave to cure.
However, I also know that the whole fuselage is not produced in one piece. Pieces of the fuselage are produced then joined together.

I would like to know the joining process works.
Since it is not fabricated as a single piece there are bound to be stresses on the joints. Will it effect/endanger the passengers?
What loads can it take since the joints will cause a drop in overall pressure (what is the maximum pressure if possible). I don't think its being welded together but not sure the reason.
Are the same methods used to join the wings?
Last but not least, are there problems due to the joints before and if so, what has been done to rectify these problems?

Damn, after reading this question, now all I can think about is what an awesome sight an autoclave would be that would allow a 787 to be built as a single monocoque structure :-D

I doubt they're welded since welding consists of softening the material until the two pieces are liquid enough to flow together and become one solid piece. This works on both metal (heat is used to do the softening) and plastics (chemicals, called glue, are used to do the softening).

@JörgWMittag now that I think about it, I'm imagining the disaster of discovering a layup issue in one small section of that 1-piece monocoque, as opposed to a problem in a section. But, yeah, an autoclave that huge would be a sight to behold!

There is a YouTube video showing the method for creating the structure:
YouTube: 787 Composite Construction
This video from British Airways shows the joining process:
Fuselage Section Joining Process which you can see is not welded, but riveted together. The sections are lined up and riveted together using joining plates.

Since it is not fabricated as a single piece there are bound to be stresses on the joints. Will it effect/endanger the passengers?

No, not more than any other conventionally constructed fuselage. The joining plates are metal and the structure around them is a metal bulkhead. The 787 fuselage is not a 100% composite construction.

Are the same methods used to join the wings?

Yes, if you watch the second linked video in full, you can see the composite wings being bolted to the fuselage structure. The entire wing support structure looks to be a conventional metal structure.

Last but not least, are there problems due to the joints before and if so, what has been done to rectify these problems?

There are new challenges with any type of new construction technique, here are a few articles that outline the challenges:
NY Times: New Challenges for the Fixers of Boeing’s 787
MIT Technology Review: Boeing's Composite Problem
Boeing has said that the 787 is as easy to repair as traditional aluminum skin aircraft. One case in point is the Ethiopian Airlines aircraft ET-AOP damaged by fire. The procedure to fix it is to cut out the carbon fiber material and put a "cure in place" patch over the hole. In this case other structure had to be replaced or repaired like stringers and bulkheads. This was the first major airframe repair that Boeing had to undertake. ET-AOP took 18 months to repair before being returned to service.

That would actually be rivets, not bolts.

@Ron Beyer
you mention about the metal plates joining the 2 CFRP components together. Care to elaborate about "joining plates are metal and the structure around them is a metal bulkhead". Does this mean the order is CFRP component, metal plates, CFRP component? or is it CFRP component side by side and there is a joining metal plate in between.

@TomMcW You are right, and I was half right :) The wings are bolted, the fuselage sections use rivets.

@user20738 I will add some detail tomorrow if nobody gets to it first. The hull sections are a carbon fiber skin laid over an aluminum substructure. In the entire fuselage, the 787 is 50% composite materials, not 100%.

Lol. I didn't watch past the first minute or so. Wanted to see if they really used bolts on the fuselage. That's a helluva riveting machine!  +1 btw

I would be surprised if they use anything but Hi-loks on the fuselage.


text:
How far away from the airport is the descent for approach and landing initiated?
I am wondering if there is a rule-of-thumb formula for the speed, altitude and the distance from the airport that the descent for approach to land is initiated? I see airlines do it about 25 to 30 minutes before landing. When, say a King Air, TBM, etc.,...doing 275 Kts at 28,000 feet will start the decent for landing at a sea-level airport, barring unusual weather or traffic conditions?

My question was marked as duplicate by 5 people...I am interested to know how was I supposed to know if my question was asked before, thus duplicate or not? There is no search box on this site that I can see. If it was duplicate, where are the original ones? Could you supply a link?

As J. Hougaard said, 3 nautical miles per 1000 feet is the general rule. However, large aircraft that can vary greatly in weight may vary that figure. Also, best to factor in whether tailwinds or headwinds in the descent.
In 747-100/200 freighters in the 1990s, we used the 3 nautical miles per 1000 feet most of the time. However, if we were heavy, i.e. we would be landing right up against the max landing weight, we would use 3.5, and especially if we would be descending in a tailwind. If we were light, in other words no cargo, 2.5 was a good figure to use. And, of course, you didn't have to use exactly 3.5, 3.0 or 2.5. What I often did was figure the 3.0 distance and then just add or subtract 10 miles or so.
In one memorable instance, I used 2.0 and it worked, but just barely. I had heard that if you had an empty aircraft and used a maximum drag procedure it could be done. I only had a chance to try it once going into Hong Kong's old Kai Tak airport on a clear day with no traffic, straight in to rwy 31.

The widely used rule-of-thumb says that you need 3 nautical miles (NM) to lose 1000 feet of altitude. So an airliner cruising at 36.000 ft will begin its descend around 110 NM before touchdown, whereas a smaller turboprop airplane crusing at say 18.000 ft will begin its descend about 55 NM before touchdown.
The speed used during descend and approach varies greately as it depends on the type of aircraft.


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How should I deal with a cable break at low level during a glider winch launch?
What should I do when rope breaks at 220-360 ft [75-120m] when there is not sufficient runway to land ahead and not sufficient altitude to make a return manuover and land with the wind direction?

Do your best not to injure yourself, there is no one answer for this. Basically comes down to choosing a landing spot that gives you the best chance of survival. You've only got a few seconds to decide, make it quick.

75m are enough to make a 180° turn and land at the airfield in opposite direction. So push the stick to gain enough air speed, pull the yellow knob to release the remaining rope, turn 180° and do a rear wind landing.

What type of terrain is past the end of the runway? Where I flew gliders, there were paddocks ahead that were suitable to land on in an emergency.

200ft should be sufficient for a return.  We practiced aerotow rope breaks at about 200' and returned with room to spare.

Had it happen once, even lower. glider had next to no forward air speed and crashed into the ground.
Luckily nobody got hurt, and the damage to the aircraft was minor.

Do an outlanding on a field.

Thanks for your answers, I think I'd rather land ahead somewhere nearby airfield as in this option has the smallest number of question marks. I've tried it recently - with ~90m alt when cable was broken, with return option, with 10 m/s wind - downwind landing,  and I ended up 25cm before a fence on the end of an airfield :) I guess The wind has a major role here, If the wind is rather strong I'd try landing ahead, if the wind is weak - try turnaround

Hey, you should as some instructor for demo and training flights. Downwind landing with 10 m/s wind feels like not good idea... Well, I am trainee myself, but in such case, I would do 270 to 360 turn. I recently was practicing this: https://www.youtube.com/watch?v=KUHwyoYKhwo

Depends on the wind and the surrounding area. If there is enough headwind, you will be surprised how short your approach will be in a proper sideslip. Practise sideslipping at altitude, so you know what to do in an emergency! Otherwise, look around for an open field where to land; gliders are easily disassembled and carried back to the airport.
When the cable breaks, push the stick forward. Be prepared for dirt flying up and into your eyes if you push hard enough! Then stabilise at the minimum sink speed, release the remaining cable and look around for your options. There should be some:

Land straight away if headwind plus sideslip allow for a steep descent.
Fly straight a bit, then turn around 180° and land opposite to your direction at take-off. Do this only when there is no or almost no wind! 100m of altitude is plenty enough for this trick.
Keep flying straight and land on an open field near the airport.
Fly a triangular, shortened pattern and land in the regular direction.

If the airport is surrounded by forest or houses on all sides, try to land on the glider port - normally, the altitude bands required for the landing straight on, the 180° return landing and a landing from a proper pattern overlap enough so that there is never an altitude that would not allow to land on the airfield.

Minimum sink speed is too close to stall speed! I would recommend at least 15 kn over stall speed in modern glass glider (in our club we fly approach with 55 kn on ASK 21).

@Sławosz: I agree, with a high aspect ratio wing the minimum sink speed can only be flown if the c${L{max}}$ of the airfoil allows it. But gliders are designed to spend quite some time at that speed, and speed stability is great at low speed. Once you have a couple hours thermalling behind you, minimum sink speed is safe to fly.

Kampf: I would never recommend flying minimum sink speed during emergency, when you are close to the ground. Even if you are good with thermaling, workload so low is so high, that you might easily stall. And since you probably want to do step bank turn, your minimum sink speed becomes stall speed (think 35-45 degree bank). Any misuse of controls can provoke spin, and during windy conditions you might easily stall because of gust and wind shear.  I recommend this page: https://members.gliding.co.uk/bga-safety-management/safe-winching/

@Sławosz: The minimum sink speed goes up with the load factor. Yes, you need to speed up for a turn. But you thermal with a load factor >1 as well. Again, after a few hours of thermalling you should be proficient, and flying at minimum sink speed gives you much more time to react. This decreases work load. A lot!

I would not compare thermalling and emergencies here. During thermalling you have much better feeling of glider, you know how it behaves, and besides, you have lot of space to perform recovery. Flying with minimum sinking speed during cable break emergency is dangerous and you should never ever do it and never recommend it to anyone, especially on internet. Low level stall/spin accident is one of most popular accidents that kills glider pilots. Cable break emergency requires training and automation (at least on small airfields), you should never launch without clear plan in mind.

It depends very much on your field which is why (in the UK at least) you list Eventualities (usually cable breaks) as part of the CBSIFTCBE pre-flight checks.
Usually you have Low level - land ahead, higher level - part turn and cross field landing, higher again-turn and reciprocal landing and higher still-run a truncated circuit and land as normal.
My main field has crops or turf fields ahead depending on wind direction so I have a lot of scope for a forward landing, it is a wide field with two main strips and a short diagonal strip so a cross field landing is possible and it is a long field so a reciprocal landing is straightforward.
Your best bet is to ask advice on your specific field from the CFI.

Your post could really do with some formatting, I found it quite hard to read, otherwise it's a good answer.

Much better with some line breaks, thank you!

when there is not sufficient runway to land ahead and not sufficient altitude to make a return manuover and land with the wind direction

If this is really case, I would not launch in such site. But since you have winch there I assume its fine - maybe its just your feeling?

~90m alt when cable was broken, with return option, with 10 m/s wind - downwind landing, and I ended up 25cm before a fence on the end of an airfield

10 m/s is a lot ( 20knts, 36 km/h), if you manage to do tailwind landing, it means you probably could land ahead as well...
Anyway, BGA has great sources about safe winch launch: https://members.gliding.co.uk/bga-safety-management/safe-winching/


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Why can a Pilatus PC-24 land on unpaved and short runways like no other business jet?
In 2013 Pilatus Aircraft unveiled their latest aircraft, the PC-24 light medium jet. On the website of Pilatus Aircraft they give several reasons why the PC-24 is better than any other business jet, the first two being

No other business jet can operate from runways as short as 2,690 feet (820 m).

No other business jet has been designed to operate from paved and even unpaved surfaces.

This sounds pretty impressive to me. But what’s different between a PC-24 and any other light medium jet on the market, which allows such short landing distances and the ability to touch down on unpaved runways?

Press picture from Pilatus Aircraft which shows the landing on an unpaved runway.

Related: How are modern jets modified to takeoff/land on a dirt runway?

I just want to know the facts about the PC-24, because I'm interested about this type of jet.

Keep in mind that the PC-24, like the PC-12, is an aircraft designed for multi-mission versatility.  Pilatus markets these aircraft to the military, emergency services, taxi, and business market, unlike other jets that are designed mostly for business use.

Well, in short, "like no other business jet" is just flat out wrong.  There are other business jets (like some of the Dassault Falcon jets) that are indeed certified to fly in and out of unimproved and short runways too.

https://en.wikipedia.org/wiki/Yakovlev_Yak-40 This plane has been operating from paved and unpaved runways for half a century now. She does require 850m of runway, however, so yeah, 820m is a real breaktrough. Until, of course, the two-engine upgrade kit for the Yak-40 becomes available, when it will be able to operate from a 750m runway. Test flights are already well underway. Some marketing people know no shame.

The Wikipedia page on the Pilatus PC-24 gives you some hints:

The PC-24 is a twin-engine business jet, a larger jet-powered follow-on to the earlier Pilatus PC-12. It is considered to be a short takeoff and landing (STOL) aircraft, being designed to operate from short and rough airstrips. It incorporates an advanced wing design, with a large double-slotted flap system to achieve the necessary performance, having a stall speed of only 81 knots at the maximum landing weight. Pilatus has claimed that the PC-24 possesses performance attributes which are unmatched by any jet aircraft on the market. It possesses a takeoff distance of 820 m (2,690 ft) and a landing distance of 770 m (2,526 ft).

...

The PC-24 is powered by a pair of Williams FJ44-4A turbofan engines. [...] The engines are mounted on the upper portion of the rear fuselage to prevent foreign object damage (FOD) during operations at rough airstrips. Other FOD-reduction measures include the enlarged flaps and chines on the nose wheel of the landing gear. The exhaust ducts of the engines feature an aerodynamic modification, having been shaped so the outflow contributes to the thrust vector during takeoff, reducing the takeoff roll distance.

So, in summary:

Advanced wing design with double-slotted flaps
Low stall speed
High mounted engines
Large flap and main nose wheel chines
Reduced take-off roll through vectored thrust
(Thanks to Peter Kämpf) Lower Wing Sweep to add effectiveness at low speeds at the expense of cruise speed.

There really isn't a lot that is going different from other business jets other than a shorter take-off/landing roll. The design isn't dramatically different, and it doesn't mention it but it probably has reinforced landing gear.
No production PC-24's have been delivered as of yet, so its hard to tell what they look like. The test PC-24's only have 150 hours behind them so they may not have had any rough-strip experience. It's difficult to say exactly what the modifications are because they may include other things like FOD deflectors on the wheels.

You may add lower wing sweep. This makes the wing more effective at low speed but limits the maximum cruise Mach number. They claim a cruise speed of 425 kts in FL 300 which is only Mach 0.73 (if I did not screw up the math) which is very low for a jet.

@PeterKämpf Thanks, added.

Update: As of Feb 2020, 70 have been built (according to https://en.wikipedia.org/wiki/Pilatus_PC-24)


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Does slipping in turns allow higher climb rates?
Sailplanes, among others, use a cotton string to show the sideslip angle. I've often heard that while circling in thermals the cotton string should show a slight slip angle to climb faster. This effect can be observed specially at high bank angles with heavy, high span gliders like the ASH-25. The german skid Wikipedia-article has even a subsection about it; however, without references. Flying like this has also the advantage of lessening the stall risk of the inner wing.
Is the indicated sideslip angle of the cotton string real or only apparent and due to the uneven incident airflow? Has this effect ever been scientifically measured and is it used in larger airplanes with FBW and sideslip angle sensors while turning?

When the string is centered, it is an indication that you are NOT slipping (but rather, flying coordinated). By not slipping, you reduce drag. By reducing drag, you can climb faster, because less energy is wasted generating drag.

@J.Hougaard That is the point:it is said that large gliders climb better when the string is not centered. I would like to know if it really is like that and if someone has measured it.

Ah I see. I misread your question. I can confirm that I was taught the same thing when obtaining my glider license, but I did not question the scientific background :)

@J.Hougaard Feel free to edit if the question is not clear enough.

This may be because you can have a tighter turn radius and therefore stay better centered in the thermal, which would indeed lead to a higher climb rate in some circumstances.  I believe that it is aerodynamically less efficient though, so in "normal" or powered flight, it would be a detriment.

@J.Hougaard: Actually, you are not slipping if the string points slightly to the outside of the turn. Being ahead of the cg, it will show the local sideslip angle (and even exaggerate it quite a bit). This effect is very pronounced in two-seaters like the ASH-25.

@Lnafziger: At the smallest possible turn radius the wing is essentially vertical, so the speed difference between the inside and outside wing is small. Sideslip will not make much of a difference (but lets the fuselage add some lift, albeit inefficiently) in this condition. You also don't get a tighter turn for the same bank angle: What the sideslip adds to the inner wing by means of the dihedral effect, it subtracts from the outer wing.

@PeterKämpf Considering that you can use the rudder to turn without any bank, I don't see how you can say that it doesn't effect the turn radius....

Source: Wikipedia.
Origin of the theory
Klaus Holighaus, one of the famous glider pilots of the 70s, and himself a glider designer at Schempp-Hirth, is at the origin of a controversy when he recommended not to turn with the ball centered, and especially when climbing in thermals. Mr Holighaus did see two problems with the perfectly coordinated turn:

It doesn't reduce drag as in straight flight.
It reduces the aircraft margin for roll stability.

I'm not an expert in the field, so I'll just rely on the article by Richard H. Johnson published in Soaring Magazine and titled "Do you really want to keep the yaw string centered?"
Before going into Klaus Holighaus' theory, let have a look at lateral stability.
Roll stability: Dihedral angle
Unless you want a very agile aircraft, an aircraft must have positive stability on the three axis, i.e. if the aircraft straight and level attitude is unsettled by some event, the aircraft must have a built-in tendency to cancel the effect of the perturbation and to return to its initial attitude.
Roll stability is required to prevent the roll angle to increase automatically after a small unwanted roll input by the pilot or by a cross wind gust. The aircraft must return to a null roll angle automatically to prevent nasty consequences, including entering a spiraling descent.
Roll stability is mostly provided by the dihedral angle of the wing as explained in How does the dihedral angle work? In short: The dihedral angle uses the sideslip to create an opposite roll motion.
No sideslip: No dihedral effect
We know turning is not controlled by the rudder but by rolling the wing with the ailerons. But at some point the lowered wing aileron may also be deflected downward for turn rate control, and this deflection may happen to be excessive:

When  the  lowered wing's  aileron  is  deflected downward,  not only
  is  its  lift increased,  but also  its drag is  increased, and a
  skidding turn  will  be induced.

At the same time the lowered wing, being closer to the turn center, is flying slower than the raised one. Slow speed, high angle of attack and more drag for the aileron can induce a stall: If the lowered wing or its aileron stall, drag on this side is increased and the aircraft is likely to enter a dangerous spiral.
However in most of the cases we can stop skidding:

The skid can  easily be corrected for by adding some top rudder to
  keep  the  skid  ball  centered.

The solution implies to add drag to the rudder, which is not satisfactory from a performance perspective.
Get the best of both worlds
This is where the mild-slipping turn theory comes from: By turning with a slight slip instead of a perfectly coordinated turn:

We keep the dihedral benefit to stabilize the aircraft around the roll axis. This is safer.
The negative roll moment provided by the dihedral allows us to use less ailerons deflection, so less drag and more speed, and better climb in the thermal.
We use less rudder, so even less drag, and even better climb.

The article's author adds this explanation about improving lift with the sideslip:

Why slipping improves the climb in some gliders is not well
  understood. In generating lift, wings also generate spanwise flow,
  outward along the bottom and inward along the top of the wings. It is
  possible that there is flow separation near the tip of some circling
  sailplanes, and that slipping changes the spanwise flow enough to
  eliminate or reduce this. Separation can increase the drag
  significantly long before the tip actually stalls.

The article also enter in the details of why this is different when winglets are used on the wing. Slipping may induce a stall instead.
How much sideslip?

The optimum degree of sideslip depends to some degree on both the
  sailplane's wingspan and dihedral angle. After many hours of flying my
  16.6 meter ventus a and similar sailplanes, I find that my best overall
  circling performance and handling characteristics occur while
  the canopy mounted yaw string is deflected about 10 degrees on the
  high side of the turn (a gentle sideslip actually).

Yaw string vs ball
The yaw string is also more accurately named the slip/skid string, as it doesn't indicates a yaw angle but a quantity of skidding/slipping, like the ball.
As @ymb1 commented, the string being on the canopy, ahead of the CG, its indication is exaggerated.
In a coordinated turn, the ball of the slip/skid ball indicator will be centered while the yaw string will be already indicating a slight amount of slip. This error will be magnified with actual slip.
The author estimates the actual degree of slip in his case is more accurately about half the indication of the string. See also Peter Kämpf's comment: "exaggeration is also caused by the fuselage shape [...]".

The yaw string exaggeration is also caused by the fuselage shape: Crossflow angles are higher than the actual sideslip angle. This and the location ahead of the cg will let it show at least twice the actual sideslip angle. Bonus point for using a Schempp-Hirth cockpit in the photo!

That's a great answer! I won't, however, accept it because it doesn't answer if the airplane is really slipping.

@mns: "it doesn't answer if the airplane is really slipping": Of course it slips, that's the key point and clear in the answer, e.g. "By turning with a slight slip instead of a perfectly coordinated turn", "How much sideslip?" and "the actual degree of slip in his case is more accurately about half the indication of the string".

@mins We can't rely on the string because it is ahead of the c.g. and will show an apparent slip even if we're coordinated... don't we have such an effect on the turn-coordinator too? Perhaps I'm being too nitpicky because I would like to see measured data instead of 'about half...'. If you ask other pilots who fly the same airplane I'm pretty sure there will be some saying 'about a fourth...'

You can't have a fixed value, as it depends on the distance to the CG and the shape of the fuselage / canopy. It's also said in the question/comments. Really I don't mind of adding reputation points, I'm trying to see what could be added (by me or others) to the explanation to fully answer your question.

@mins It's not about a fixed value, of course it will be different on every airplane. It is more about qualitative feelings (better climb gradients when the yaw string goes to the side without knowing if the slip is real or apparent) vs quantitative data I would like to see (e.g. constant radius turn polars for an airplane with different true sideslip angles) to get past the popular wisdom stadium to a scientifically measured and explained phenomenon. But I guess it just hasn't been measured, so I'll accept it.

@Gypaets -- just came across this old answer and associated comments-- one thing that seems to have been missed in the comments, was that the Holighaus article specifically recommends leaving the slip-skid ball about half a diameter off-center.  The reading of the sllp-skid ball is not dependent on its fore-and-aft location in the fuselage.  So yes, the aircraft really is slipping when flown as recommended in the article.  Obvious the savings in sink rate is very hard to measure. It would be interesting to see data from controlled tests in still air, descending through thousands of feet.


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What are Forward flight, Straight flight, Level flight, and Cruise flight in a helicopter?
Pretty straightforward: what is the difference between forward flight, straight flight, level flight, and cruise flight in helicopters?

Forward flight is how the helicopter flies in most situations. It flies similar to an airplane in this situation, with pitching up and down to increase and decrease airspeed. This is controlled by use of forward and aft cyclic.
Straight flight is where the tail rotor is set so that it opposes the turning tendency of the main rotor. This essentially prevents the helicopter from yawing while in flight. This is controlled by using the anti-torque pedals.
Level flight is simply when the helicopter is flying at a constant altitude. This does not mean, however that its nose is pointed ahead, or that it is flying forward, merely that it is not gaining or losing altitude.
I'm not sure exactly what cruise flight is. When a helicopter is "cruising", it most likely is maintaining altitude and flying straight ahead, but I don't think there is a separate term for it.

To be pedantic:  Straight also implies in balance since it is possible to have yaw perfectly countered but to be flying anything other than straight. I still have scars on my head from being slapped by my instructor for not having the string in the middle.  Isn't it strange how someone had to point it out when you're learning but with the hours comes that natural feeling of discomfort with flying out of balance. I am guessing that cruise flight refers to the attitude and power setting required for range vs speed vs endurance?

the problem with this answer is that you can't control a helicopter with any single control. The slightest change in any input (or no input at all!) requires a change in every other input. There are no "tail rotor pedals" in a helicopter. they are called "anti-torque pedals," which corresponds to the yawing tendency you describe.

@rbp - yes, but adding that qualifier wouldn't add anything to the answer - in such a complex system, I would think it would be understood that changes in one system do affect others

the "pitch versus power" argument is over, and would have applied even less to helicopters than it ever did to airplanes. http://avweb.com/news/features/Pitch-Or-Power222778-1.html

Forward flight in a helicopter means that the the horizontal component of lift is forward of vertical, and no turning component is present. Contrast to turning flight.
Straight flight is forward flight, with the yaw string centered. Contrast to a slip or a skid, or sidways or backwards flight.
Level flight means that the helicopter maintains altitude.
Cruise flight in a helicopter is the flight regime between departure and approach, at a power setting optimized for traveling between locations.


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Why does the Yak-28 cockpit have two attitude indicators?
Why are there two attitude indicators in the cockpit of the Yak-28? Below is a picture from this YouTube video.

It's possible that the soviet attitude indicators are subject to tumble when operated inverted. the easy way to overcome this is to provide two instruments, one of which does not tumble when flying mostly right-side up and the other does the same for flying upside-down.
Soviet-era acrobatic planes had dual G-meters installed for this same reason. One was installed upside-down and registered negative G's while the other was installed right side-up and registered positive G's.

This is an interesting answer but I'm not sure that it's possible to design an attitude indicator that is more prone to tumbling in upright flight than inverted flight, or vice versa.  Isn't it a rapid CHANGE in attitude that tends to cause a tumble?  If one the indicators was biased toward tumbling in upright flight, wouldn't it likely be already tumbled before the pilot began transitioning to inverted flight?  Could be a basis for another ASE question here about exactly what maneuver tends to cause an (older design) attitude indicator to tumble.

Many of the modern military aircraft I have seen have two Attitude Indicators for redundancy. Although, one may be in the HUD. GA aircraft with glass panels certified for IFR also have two for the same reason.


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Which fighter jet is the safest outside of combat?
I've heard that the Bloodhound SSC project decided to use a Eurofighter engine, because this was "safe". This got me wondering if there are any numbers on which fighter jets are safest. Something like a non-combat fatalities to flight hours ratio?

what research have you already done on non combat losses to fighter aircraft?  (Your idea to do it per flight hour is a good one; it's how the Naval Safety Center (among others) gages accident rates)

As long it is not the J79-GE-3A...

Any unmanned remote control or autonomous drone seems like the safest possible jet to me, but I'm sure that's not what you were looking for...

It's possible the Eurofighter engine was considered safe compared to something other than contemporary fighters. They had a choice: either a factory-supported, almost-new EJ200 (form the only air force likely to support a British land speed record attempt), or a random used jet engine stripped out of a retired aircraft.

There are a number of barriers to labeling a single fighter as the "safest" - your proposed method is probably the best overall picture, but there are some barriers to even that.
Differences in use and training
Fighter aircraft are extremely complex and are asked to performed complex missions, even in training, where there may be little room for error. Due to the United States' huge military budget and wide-reaching foreign operations, US military pilots tend to have high flight time compared to many smaller countries, including other ones that fly US-produced aircraft. (Russia's modernization has led to increased flight time there, though still far below US or Cold War levels. China's modernization has pushed their training levels much closer to the US, as noted on p. 10 in this 2016 study ).
As pilot experience leads to decreased accident rate (to a point, at which accident rate goes up again even higher than inexperienced pilots), some of the air forces which fly very little likely have some contribution to a possible higher accident rate, irrespective of the safety of the aircraft they fly.
In addition, differences in philosophy of use from one country to another may lead to more dangerous employment - high speed low level flight is inherently more dangerous than flying high-altitude caps - and two air forces may utilize the same aircraft in such vastly different roles. This may lead to a fatal accident from a malfunction that at altitude, would be comparatively benign.
Lack of records
This is the biggest barrier, I think. Unlike commercial airliners, which worldwide predominately are produced by only two major manufacturers (Airbus/Boeing) and a handful of smaller regional-class producers, fighter aircraft are produced by a number of companies:

USA - Boeing, General Dynamics, Lockeed Martin
Sweden - Saab
Britain - BAE
Europe (multiple contributors) - Eurofighter GmbH
France - Dassault
Russia - UAC (combination of former companies Mikoyan, Sukhoi, Yakovlev, and others)
China - CAC, XAC, HAIC, Shenyang
India - HAL

While some of these aircraft are unique to a single country, many are used by multiple countries (e.g. for US-produced aircraft, numbers may be available for the US accident rates, but not for other countries). The F-16 has had 4,500+ aircraft produced, and is operated by 26 different countries on 5 continents. One difficulty with this, other than the difference in records is the number of different variants in service. As Mazura noted in a comment below, the F-35 has seen no fatal incidents in US service, but other countries have had incidents. 9 countries currently operate a F-35 variant, with at least 4 more countries planning to acquire the type. The MIG-29 has at least 24 current and 10 former operators spanning 5 continents as well. So great difficulty exists in retrieving fleet-wide numbers (in addition, as political situations change, it may make spare parts acquisition or maintenance more difficult, leading to major variances in reliability).
In addition, countries with smaller militaries that have traditionally relied on importing their military aircraft such as Japan, Israel, Turkey, Pakistan and Taiwan have produced or are developing indigenous fighters.
Due to the diverse nature of these countries that produce the aircraft - some of which are produced by somewhat state-owned companies, it makes the retrieval of records difficult.
Additionally, since Airbus and Boeing need private corporations to purchase their aircraft, and safety is a huge selling point (or rather, poor safety record is a major turn off) in selling tickets, then the easy retrieval of accident histories for airliner class aircraft is much more crucial than high-performance military aircraft.
Age of Fleets
Looking at probably the best kept public record producers,  The U.S. Air Force Safety Center has year-by-year and lifetime mishap histories for all current USAF fighters, but it doesn't separate out combat and non-combat incidents/fatalities. But when you compare the various US fighters:

A-10: 51 fatalities, 5.65 million hours (0.90/100,000 hours)
F-15: 44 fatalities, 6.80 million hours (0.65/100,000 hours)
F-16: 86 fatalities, 11.28 million hours (0.76/100,000 hours)
F-22: 1 fatality, 355 thousand hours (0.28/100,000 hours)
F-35: 0 fatalities, 96 thousand hours (0.0/100,000 hours)

But the F-22 has only been flying since FY2003, and the F-35 since FY2012, so comparing those to aging aircraft that have been around since the early 1970s doesn't paint an accurate picture. You can look at the 5-year and 10-year trends, however to see a more recent summary. For the USAF, the best rate would be the A-10, with no fatal accidents in the last 10 years, averaging 88,000 flight hours a year. Not unique to the USAF (indeed, all aviation sections of the US Military deal with this), many other countries have dealt with aging fleets as well, often due to the less frequent production of new designs since the cold war. (ex: Russia, Iran).
One note is that many combat aircraft in service with even the most modern air forces are no longer in production - the A-10 was last produced in 1984 and the MIG-31 in 1994, while some aircraft (like the MIG-29 and F-16) are still in production today.
Conclusion
Due to the barriers listed above, I think objectively calling a single fighter the "Safest" is likely an impossible task. But from the various characteristics, the safest record probably belongs to an aircraft that is flown by a country fairly regularly, leading to good experience in its pilots, possibly is employed predominately in an air-to-air role more regularly, and is produced more recently, yet long enough ago that the major design flaws in a new aircraft are worked through. I would say the best candidates would be the F-15E or the Eurofighter (which I found an article indicating 2 deaths in 240,000 flight hours across a diverse group of countries - Wikipedia indicates another 4 fatal crashes since then in an unknown amount of flight hours).

You correctly noted the issue with new designs. For example if the F-35 had had one fatality for some reason it would be the worst performer in the table not the best. But it is possible to ask statistical questions, like "given this data, what is the probability that the F-35 is less safe than the F-16, assuming that fatalities are random and not from some systematic cause". That is probably beyond what the OP is looking for, though.

Lockheed Martin F-35A Lightning II - Fatalities: 1 - https://aviation-safety.net/wikibase/223829 "assembled by Mitsubishi" doesn't count?

@alephzero. That actually sounds like exactly what OP might be looking for.

what is the reason why very experienced pilots have higher accident rates than inexperienced pilots?

@mazura - i meant to clarify that in my second point, but since some aircraft are operated by widely varying services (F-16, MIG-29, Flankers, now F-35) it makes tracking down the by-jet numbers very difficult

@Michael - mainly complacency - once a pilot becomes comfortable in an aircraft, they tend to make mistakes due to inattention or complacency while inexperienced pilots tend to make more alert for their own mistakes, and make mistakes due to lack of knowledge

Actually, there is one fighter jet that stands out from the rest with regard to its safety record, the Saab JAS-39 Gripen.  It has flown for 24 years (32 if you include its first flight and development), is in service with five air forces in all climates and close to 300 have been built.  In all this time it has NEVER suffered an engine failure and has only one fatality (back in 2017).  Moreover, that one fatal crash was not the fault of the plane itself according to the investigation conducted by the Royal Thai Air Force.  This pretty much debunks the myth that twin-engine fighters are safer than single-engine fighters.
No other fighter aircraft in history has so spotless a safety record as the Gripen. In fact, I'm pretty sure that none have even come close.  It is literally the safest fighter aircraft ever produced.

This is a great point, but if you could back up the numbers with links that would make it a lot better

https://en.wikipedia.org/wiki/Accidents_and_incidents_involving_the_JAS_39_Gripen

With the frequency of edits, @ghellquist, linking to Wiki isn't really useful. Putting the numbers into the answer (and quoting the source) fixes them in time, should Joe Random go edit the entry...


text:
How to lean with constant speed prop on multi-engine aircraft without EGT
This question is more about the indication of engine roughness. The procedure for leaning without an EGT is clear in the POH and Lycoming documentation which basically states as follows:
Best power:

lean until "engine roughness is noted."
Move mixture forward approx. 1/2 inch toward full rich.

Best Economy (75% power or less):

lean until "engine roughness is noted."
Enrich mixture until engine runs smoothly.

My question is what is engine roughness as defined by Lycoming in this situation?
I like to lean it until engine is clearly sputtering (no question its running rough), before I enrich, but most CFIs I've flown with lately seem to "detect" roughness with some magical senses, I think they are just going with their gut as opposed to empirical indications such as drop in MP or objectively detectable drop in engine performance.
My understanding of "engine roughness noted" would be a clear audible roughness of the engine being leaned, along with a slight, yet detectable drop in manifold pressure of said engine.
Engine Roughness is not a defined term in the POH or Lycoming documents I've read. So in my opinion it should be referring to a clearly objective condition that any person can easily identify.
I'm hoping someone can confirm or provide some objective guidance on how to identify "engine roughness" when leaning an engine on a twin with constant speed props.

I would not lean to "sputtering", you are asking for in-flight failure (as in engine stops, maybe not damage) there. You should be able to hear/feel the engine getting rough. When I do this, I can notice the increased vibrations and differing engine tone.

CFIs do it (and much more full cut-off) all the time in multi-engine aircraft to simulate engine failure and to actually shut down the engine. Although I can appreciate what you're saying.

I've never had a CFI pull the mixture knob out to simulate engine failure, I'd probably have a long talk with the CFI and not in a good way. Yes, you pull the mixture knob to stop the engine, but you don't want to get so close to shut down in flight that your engine is sputtering.

In a twin I find it difficult to know for sure what I'm "hearing/feeling" is coming from the engine that it is supposed to be coming from, plus the twin I've been flying recently has a bit of a wandering RPM that continuously goes in and out of sync. At cruise altitudes (<75% power), there isn't much damage you can do by leaning momentarily until engine runs rough and cough a couple times.

@RonBeyer interesting, I'm going to have to go back and review that. Thanks for the info. I'm thinking back and I think they have always pulled mixture to simulate engine failure on me. How do CFI's simulate engine failures where you are?

@RonBeyer I agree with you with regard to simulated engine failures in single engine aircraft--you don't want to pull mixture in a single. However, what I was talking about is that in multi-engine aircraft, CFI's simulate engine failures by actually putting mixture to cutoff. I have seen POH recommendations for power and RPM settings that simulate engine in feather, but that is usually used to practice single engine landings. At altitude, every CFI that I can remember has just reached up and pulled the mixture back.

My point with regard to CFI's pulling mixture out to simulate engine failures at altitude, is that in twin trainers (like Piper Seminole) this is done to the engine 50+ times per day. So repeated and aggressive (and abrupt) leaning by CFI's at altitude appears to cause no ill effect to engines on twins, so my conclusion that a very slight leaning by me until hearing a definitive audible roughness (a cough or two) isn't going to hurt anything. I guess this is a little off topic. :)

There is a big difference between "by feel" leaning of a carburated vs fuel injected engine.  Carburated engines have less even fuel distribution and when leaning, one cylinder, the already leanest one, will start to misfire quite a bit before the others.  So with a carb it's easy to detect one cylinder starting to misfire (the cylinder usually doesn't totally quit right away - you will pick up random misses in the vibration rhythm).
With a fuel injected engine the mixture distribution is more even and misfire of one cylinder will be accompanied by misfire of its sisters almost immediately.  So on an injected engine you will lean and suddenly the engine will really start to stumble, whereas with a carburated engine there will just be these random dropouts in the engine's "beat", getting worse as you continue leaning.
So if you are leaning on an aircraft with fuel injection it is really fairly straightforward.  The engine will go from smooth to pretty rough right away, whereas with a carb it takes careful attention to detect the early onset of misfire. You are at a disadvantage to some degree because the airplane is less familiar than to the instructors.  It's a skill you will pick up over time as you develop at ear for it.
Don't worry too much about it. Err on the side of smoothness, knowing that the difference in fuel consumption between perfect and good enough is negligible (and if you're paying hourly on a hobbs meter, there is no benefit to you anyway), and it's wise to err on the side that is beneficial to the engine (too rich).

Good point about carburated engines, they'll complain a lot before shutting down.


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At what speed will a flat iron square of 10 sq meters fall at every second if dropped from a height of 10 meters?
I've been trying to read the Wikipedia articles on drag, and picked up some books on physics to scan, but they're almost unreadable for me, the average person.
What are the equations for the speed and acceleration of a flat iron square, 2 cm thick and 10 sq meters in size, falling parallel to the ground will fall at every second, taking into account gravity and drag?

Do you mean per gravity effect only (the speed increases by 9.81 m/s per second), or taking into account aerodynamics (drag as you mentioned)? If the latter, then what is the thickness?

taking into account gravity + drag, thickness is 2 cm

Do you want to assume that the plate stays parallel to the ground? Intuitively, at least to me, it doesn't seem that would happen in the real world.

I'll do that then.

Added it as question there.

Um, I think the moderators are supposed to migrate it rather than double-post.

I think you really do not need to care about aerodynamic drag in this particular case. The height of drop is so low and weight of the plate so heavy in relative terms that the plate would hit the ground long before you can say word DRAG. It would be a more interesting question had you been dropping the plate from a height of say 1000m. Then perhaps we might see the plate reaching terminal velocity before it huts the ground due to aero drag.

The event will take longer than you think.  Even with no drag, an object will only fall 4.9 meters in the first second.  It will take about 1.4 seconds to fall 10 meters.

Yes. I can calculate that. But obviously that time is not sufficient to demand consideration of drag for any practical purpose. For academic interest one can always calculate some numbers which may not be of much relevance in practice.


text:
Can you help me identifying this WW1 German or Austrian airplane from my photo collection?
Can anyone help me to identify this WW1 aircraft?
It looks as though it reads Otto No. 202 on the plane.

@DalamaniSingh is correct, but the people who built and flew this plane never called it "Otto pusher".
Gustav Otto was one of the seven children of Nikolaus Otto, who designed the first practical internal combustion engine and after whom the Otto cycle is named. He started an aircraft factory in 1909 near Munich and later relocated north to Oberschleissheim. The first product of this factory was simply called "Otto Doppeldecker" (Otto biplane). It was produced from 1912 to 1915, but was obsolete by the time WWI started. The plane was mostly used for training. The factory had to close in 1915 and the historic building now houses a collection of aircraft and has become a museum. In the attached restoration workshop, a replica was built recently.

This is a world war 1 plane Otto Pusher which was operated by the Germans. Here is another image:


text:
Are modern airliner avionics spill-proof?
I thought about this today, and didn't find much other than a flight decades ago that made an emergency landing due to spillage causing the trim wheel to uncontrollably pitch the plane down into a dive. How spill-proof are avionics today? Especially the centre pedestal? I can't imagine it happens often but it has to happen from time-to-time where a pilot will spill their coffee on the centre pedestal right? It seems like it would be an oversight to not have these be spill-proof.

3 fatalities in that incident, yowch.

Must have been a rough dive if they regained control at 1000ft from cruise. I wonder how the fatalities occured

@eleventyone missed that deet, thanks!

@Bianfable yes it does, searching didn't return that somehow but thank you!

The descent was from FL230-ish, so it may not have been cruise (it's not clear which taxonomy is in use in that report). It's still a long way to fall mind.

Apparently not: http://avherald.com/h?article=4c3da991&opt=0

The actual avionics are down below. The units on the center console are just control heads mostly, although they are still boxes with electronics in them.
In new design these days, there is some effort on the design side to make the units coffee resistant, by using sealed switches and rotary control devices, etc.  But they aren't completely water tight.
The components on the lower center console live a difficult life, getting splashed and kicked regularly by coffee spills and people climbing in and out. On smaller cockpits the sub panels near the bottom of the center console close to the seat get replaced quite a lot from getting kicked.
I participated in a project to approve a new switch design in an audio control panel on the CRJ900, which was right in the coffee target zone, and the switch had to be a water resistant design.
Avionics down below, if they are under the passenger cabin, can also get wet from spills in the cabin that migrate down under the floor.  On the CRJ program, certain black boxes had little rain shields incorporated above them to protect the boxes from getting dripped on.  This is expecially bad for components under the floor near entry and service doors where rain can get in and find its way under the floor and run along floor rails to drip some distance from the original source.


text:
What does AO2 in the remarks of an ASOS/AWOS mean?
I occasionally see AO2 in the remarks of an automated weather report. What does the AO2 mean?
METAR KSLC 280453Z 00000KT 10SM FEW042 SCT055 BKN080 04/02 A2981 RMK AO2 CIG 037 NW RNWY SLP087 T00440022

The AO2 notation means the station is automated and has a precipitation
discriminator. AO1 means automated but no precipitation discriminator. The precipitation discriminator will be able to sense the difference between rain and snow.
The METAR format is documented in the Federal Meterological Handbook No. 1 in Chapter 12. This particular notation is:

Type of Automated Station (AO1 or AO2). AO1 or AO2 shall be coded in all METAR/SPECI from automated stations. Automated stations without a precipitation discriminator shall be identified as AO1; automated station with a precipitation discriminator shall be identified as AO2.


text:
No alternate destination when flying from Oakland to Honolulu: is that correct?

When you file a flight plan from Oakland to Honolulu
  you put "Glory" as your alternate destination.[1]

Is that the correct way to file the flightplan?
Scanning back a few paragraphs, the airplane in question was a Cessna 402.
[1] B. May, Under His Wing: Adventures in Trusting God, p. 40 (Multnomah Publications, 1991. ISBN 0-930014-94-4.)
Please note while the publication is 1991, the actual copyright is 1979, so we are looking before that date.

Can you clarify the context and what it is that you're asking exactly?

I'm suspecting at the time of the incident there was only one airport in Hawaii, or alternatively there really aren't alternate destinations that are likely to be any better off.

@Joshua Hawaii was a significant location for the military during World War II: there were plenty of air fields there. What are now Hilo and Honolulu International Airports both opened in 1927.

So anyway I'm not sure how to reword the question; however the correct answer is certainly spread between Davis Richerby's comment and Mike Foxtrot's answer. I'm ready to conclude overwritten for effect, but essentially true. If trouble a long way over the water, if cannot make Hilo, doomed.

I'm not sure exactly how to plot a great circle route; however I'm pretty sure Hilo is not much closer than Honolulu.

@Joshua You can plot a great circle route at gcmap.com. Also, there are more airports in Hawaii than just Hilo and Honolulu. Also also, you need an alternate for reasons other than failure of your own plane. The weather at your intended airport, or an incident there may force you to land your perfectly working plane somewhere else.

@DavidRicherby, would the weather not be the same as ALL airports within range of Hawaii?

@IanRingrose Not at all. Hawaii is mountainous and 400 miles long. As a simple example of the variability of its weather, Honolulu receives 3,000 hours of sunshine a year and 430mm of rain, whereas Hilo gets 1,800 hours of sunshine and 3,200mm (sic) of rain.

There are plenty of alternative airports in Hawaii, but none in-between of course. So what the author is getting at is that you're probably going to be dead if you run into trouble.
While I can't say I know anything much about the Cessna 402, it apparently has a normal range of around 1,273 nm (1,467 mi, 2,360 km), compared to a distance Hawaii-Oakland of 2050nm (2359mi, 3797km). Hence, you're going to have to install additional fuel tanks and fly pretty heavy.
There a few considerations if you get a problem:

You will have to descend since the engine can't generate enough thrust at that cruise altitude. I found a source stating the ceiling for both engines is around 26180 ft compared to 11320 ft for one engine.
The fuel consumption increases in the remaining engine and will be greater than the total for both engines for the same flying conditions.
You're going to have to run the remaining engine hard for the remain of the flight.
If your temporary ferry tanks screw up (or any of the normal ones for that matter as well), you won't make it.
You may damage/break the gear if you have to return to the airport overloaded. "I once saw a Cessna 402 parked on the ramp at Honolulu with the gear punched half way up into the wings from a poorly executed overweight landing." Source

I can't do a calculation on for what portion of the flight you would be 'screwed' but I imagine it might very well be a bit of the journey. Several people seem to be in the opinion it's a pain to fly heavy and difficult to with one engine.

Well the book is really scant on details, but it mentions an extra ton of fuel.

The good news, as I recall, is that there are headwinds to the halfway pint, and favorable winds on the second half.

Another bit of trivia is the Bob Hope lived in Hawaii and flew back and forth in his 4-engine JetStar

The first two points should be in reverse order. Developing the same total power with one or two engines needs about the same amount of fuel. It is the lower ceiling that causes the higher fuel consumption, because there is more drag at lower altitude and thus more power is needed.

@JanHudec although I'd agree that it's not a huge difference, one engine at higher power is usually less efficient (in terms of miles per gallon, not gallons per hour) than two running at lower power, simply due to the lower speeds involved, producing less waste to heat/noise.

@JonStory: For pistons, slightly yes. For turbines, however, definitely false. Turbines (both prop and jet) are most efficient at their maximum power.

@JanHudec I haven't seen many jet powered Cessna 402's though....

@JonStory: Of course. Still the need to fly lower is and the corresponding higher drag is the larger part of the reason why more fuel is used, even with a piston engine above it's most efficient power range.


text:
Where can a PDF of the TPP legend and supplemental materials be found?
When you buy a book of government approach plates, or a Terminal Procedures Publication (TPP) it comes with a legend and other supplemental documents along with it. Now that a lot of pilots are using digital flight bags like ForeFlight it's easy to overlook this stuff.
What I need is a PDF of these materials, but after a lot of searching I'm just not finding it. Does anyone know if the FAA publishes this digitally? This page mentions that it's available, but I don't see the download for it.

Apparently the file you are looking for is FRNTMATTER.PDF, located in the "D" download file. DDTPPD_201603.zip
(but it still looks like you have to download the 900MB ZIP file just to get that one important file)
The file that @JonathanWalters referenced has very similar information in a much smaller download and a different format.  But there are differences between the two.
I recommend downloading both the Chart Users guide at the FRNTMATTER.PDF file, and judging for yourself which one (or both) best has the info you are after.

Downloading this huge file is not necessary.

Was the down vote really necessary?

I'm assuming that @JonathanWalters downvoted me because he found a smaller download that has the same info.   Thats fine.   I still consider my answer valid and correct, even if it is less than optimal.   Thanks for the upvote as well.   (The Voting Lords giveth, and taketh away....)

Your answer is valid to be sure. I think this question is properly answered only by the combination of the two answers.

FRNTMATTER.PDF can be downloaded independently at http://www.aeronav.faa.gov/d-tpp/FRNTMATTER.PDF

Much of the information is available on FAA.gov, in a handy web-based format in a document called the "Aero Guide", or as a PDF download called the "Aeronautical Chart User's Guide".
The Aero Guide provides a digital document of the reference information provided with both the VFR and IFR paper aeronautical charts and publications, as well as other information not included with the charts. For example, much (if not all) of the same information on Approach Lighting Systems is included in the Guide. However, some other information found in the TERPS publications is not available, such as the Inoperative Components Table.
Here is a screen cap of one of the legends available in the Aero Guide on the TPP Symbols tab (or in the PDF):

For a web-based version follow the link to the Aero Guide and click on the TPP or TPP Symbols tabs for the TPP information you want.
For the PDF: as quoted from the 'product description' on the Aero Guide page:

Provided here is the Aeronautical Chart User's Guide in a web-based format. This web-based version is designed to easily reference chart symbology while providing access for all devices. For now, the downloadable PDF version is still available at the bottom of this page.

That link to the downloadable PDF is found at the bottom of the Intro tab. This is the current direct link. Go to the bottom of that page (although not all the way down into the generic FAA.gov info) and look for the "Aeronautical Chart User's Guide" link. This document is about 22 MB in size.

I'm familiar with the Aeronautical Chart User's Guide, but there are a lot of things it lacks that the legend has. How to read the ALS section is one example.

If you're looking for Approach Lighting System legend, you'll find it around page 16 of the file mentioned in my answer


text:
How to replace two propellers on the same aircraft reaching the time limit?
When both propellers of the aircraft reach the time limit (60 month) and need to be replaced, can both propellers be replaced at the same time and perform only one flight test to reduce time on ground or should they be replaced one at a time just for safety or to investigate any discrepancy that may arise?

It's a useful idea to include a details on the country and legislation, since these can change from area to area and helps to narrow the down possible answers for the type of aircraft you are interested in :)

I can't name a source now since it is quite a while since I've seen it, but I believe to remember that at least in the EU and Switzerland this does not work. Testing a propeller implies a high probability of it failing thus you need the other one to already be tested to ensure that at least one is most likely working with no problems. Therefore you would have to replace one, test it, then replace the other one and test it. If I can find the source again, I make it a full scale answer, till then I hope somebody else can point you in the right direction.

Why is a flight test needed for a prop change?

@SteveH That sounds like an interesting thing to ask as a new question. :)

Any paralells to  a brand new plane with two untested props? It is probably flight tested at only one test event.

Can you please elaborate if you are asking "can" under regulations? Or out of an abundance of safety

Since replacing a propeller with the same propeller that is in the Type Certificate is not a major repair, the FAR(s) (CFR Title 14) that most apply are 43.7 and (assuming the airplane is under Part 91) 91.407.
43.7 states:

(a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in §43.16. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. If special equipment or test apparatus is recommended by the manufacturer involved, he must use that equipment or apparatus or its equivalent acceptable to the Administrator.
(b) Each person maintaining or altering, or performing preventive maintenance, shall do that work in such a manner and use materials of such a quality, that the condition of the aircraft, airframe, aircraft engine, propeller, or appliance worked on will be at least equal to its original or properly altered condition (with regard to aerodynamic function, structural strength, resistance to vibration and deterioration, and other qualities affecting airworthiness).

So you must follow the maintenance manual and put the airplane back in a state at least as good as original. If you think that putting them both on does not qualify to satisfy this requirement, then you should do them one at a time. If it does satisfy this, then in that case, 91.407 comes into play, which states:

(a) No person may operate any aircraft that has undergone maintenance, preventive maintenance, rebuilding, or alteration unless—

(1) It has been approved for return to service by a person authorized under §43.7 of this chapter; and
(2) The maintenance record entry required by §43.9 or §43.11, as applicable, of this chapter has been made.

(b) No person may carry any person (other than crewmembers) in an aircraft that has been maintained, rebuilt, or altered in a manner that may have appreciably changed its flight characteristics or substantially affected its operation in flight until an appropriately rated pilot with at least a private pilot certificate flies the aircraft, makes an operational check of the maintenance performed or alteration made, and logs the flight in the aircraft records.
(c) The aircraft does not have to be flown as required by paragraph (b) of this section if, prior to flight, ground tests, inspection, or both show conclusively that the maintenance, preventive maintenance, rebuilding, or alteration has not appreciably changed the flight characteristics or substantially affected the flight operation of the aircraft.

I would argue that a propeller change cannot be satisfied by (c) above and does require a flight test. However, there is nothing that requires separate flight tests UNLESS the maintenance manual requires it, in which case part 43.7(a) from above comes into play. But as a "fail safe" a good run-up on the ground at high throttle with the brakes applied, followed by an inspection of the work can give good confidence in the work prior to the flight test.

Welcome, nice answer for a new guy, well done! The only thing you're missing is a source reference, and a good link to the source. Thanks for finding an old question in desperate need of a decent answer.

Sure thing. FARs are the Federal Aviation Regulations, which are in CFR (Code of Federal Regulations) Title 14. Pretty much any websearch will give them, I linked to one of them.

@Paul, Truly a quality answer! Welcome to Aviation.SE!

Excellent, @Paul, well done! That now nicely fits the StackExchange model, and should garner you more up votes. Since your first answer was this good, I'm certain you'll grace us with many other excellent answers, as well.

It would probably be safer to replace one at a time in case the propellers were defective. Better safe on one good prop than sorry on two bad ones.

Just because you say so? Or are there any requirements to do so?

Check your country's rules on that, but even then it is one of the unwritten rules of flying that keeps almost everything with a fail-safe backup.